Data storage media are commonly used for storage and retrieval of data, and come in many forms, such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks, cards or tape, and the like. Magnetic tape media remains an economical medium for storing large amounts of data. For example, magnetic tape cartridges, or large spools of magnetic tape are often used to back up large amounts of data for large computing centers. Magnetic tape cartridges also find application in the backup of data stored on smaller computers such as desktop or laptop computers.
In magnetic tape, data is typically stored as magnetic signals that are magnetically recorded on the medium surface. The data stored on the magnetic tape is often organized along “data tracks,” and read/write heads are positioned relative to the data tracks to write data to the tracks or read data from the tracks. Other types of data storage tape include optical tape, magneto-optic tape, holographic tape, and the like.
As the number of data tracks on data storage tape increases, the data storage capacity of the data storage tape likewise increases. However, as the number of data tracks increases, the tracks usually become narrower and more crowded on the surface of the data storage tape. Accordingly, an increase in the number of data tracks can make positioning of the read/write head relative to a desired data track more challenging. In particular, for proper data storage and recovery, the read/write head must locate each data track, and follow the path of the data track accurately along the surface of the data storage tape. In order to facilitate precise positioning of the read/write head relative to the data tracks on the data storage tape, servo techniques have been developed.
Servo information refers to signals, patterns or other recorded markings on the data storage tape that are used for tracking purposes. In other words, servo information is recorded on the data storage tape to provide reference points relative to the data tracks. A servo controller interprets detected servo information and generates position error signals. The position error signals are used to adjust the lateral distance of the read/write head relative to the data tracks so that the read/write head is properly positioned along the data tracks for effective reading and/or writing of the data.
With some data storage tape, such as magnetic tape, the servo information is stored in specialized tracks on the medium, called “servo tracks.” Servo tracks serve as references for the servo controller. Conventional servo tracks typically hold no data except for information that is useful to the servo controller to identify positioning of a read/write head relative to the surface of the data storage tape.
The servo information recorded in the servo tracks may be sensed by one or more servo heads. For example, servo heads may be dedicated heads that read only servo information in the servo tracks. Once the servo head locates a particular servo track, a data track can be located on the medium according to the data track's displacement from the servo track. The servo controller receives detected servo signals from the servo heads, and generates position error signals, which are used to adjust positioning of a read/write head relative to the data tracks.
Time-based servo techniques refer to servo techniques that make use of servo information and time variables. Time-based servo techniques are particularly effective for magnetic tape, which typically feeds past read/write heads at a constant velocity. For example, N-shaped servo markings, servo markings such as “<<< >>>” or “/// \\\,” or the like, have been developed for time-based servo techniques. Such time-based servo information is typically formed in a servo track of the magnetic tape.
When time-based servo techniques are used, the time offset between detection of two or more servo marks can be translated into a position error signal, which defines a lateral distance of the read/write head relative to a data track. For example, given a constant velocity of magnetic tape formed with marking “/ \”, the time between detection of “/” and “\” becomes larger when the servo head is positioned towards the bottom of marking “/ \” and smaller if the servo head positioned towards the top of marking “/ \”. Given a constant velocity of magnetic tape, a defined time period between detected servo signals may correspond to a center of marking “/ \”. By locating the center of marking “/ \”, a known distance between the center of the servo track and the data tracks can be identified.
Amplitude-based servo information refers to servo information in which detection of the servo signal amplitude enables identification of head positioning relative to the medium. In other words, as the head passes relative to the data storage tape, signal amplitudes of detected servo information can be used to determine whether the head is positioned correctly relative to a track on the medium. For example, amplitude-based servo windows can be positioned above and below a track centerline to define the centerline. Amplitude-based servo information is also commonly implemented in magnetic tape media.